透明图像PHP上的透明图像

Question

I'll explain a bit of my situation.

We got an island with cities. These could be:

• Owned by my alliance
• Owned by an enemy
• Owned by nobody (free)

So we got 3 "cities"-images and an island image, looks like this:

Now we want to put these cities images on the island image. For example, we put a city owned by nobody on the island like this:

<?php
// Get image
$im = imagecreatefrompng('island.png');
imagealphablending($im,true);

// Get our "Free-city-position" image
$stamp = imagecreatefrompng('free.png');

$pos_x = 190 - 15; // Position X = 190 - the half of the free.png image = 30 / 2 = 15
$pos_y = 225 - 15;// Position Y = 225 - the half of the free.png image = 30 / 2 = 15

imagealphablending($stamp,true);
imagecopy($im, $stamp, $pos_x, $pos_y, 0, 0, imagesx($stamp), imagesy($stamp));

// Output image
header('Content-type: image/png');
imagepng($im);
imagedestroy($im);
?>

And now there is our problem: The city image is not transparent on the island image! It looks like this:

I thought imagealphablending should do the trick, but unfortunately, it doesn't.

How can we get the transparant city image on the island?

Answer 1

Taken from PHP's comment on GD functions :

If you are trying to copy a transparant image on to another image, you might assume that you should apply the ImageAlphaBlending function to the image that has the transparancy, the source image. In reality, you must apply the ImageAlphaBlending function to the destination image. Basically it's saying, "make the specified image respect transparancy".